Semiconducting materials find uses in many applications. For example, semiconducting materials can be used in electronic devices as processors formed on semiconductor wafers. As a further example, semiconducting materials may also be used to convert solar radiation into electrical energy through the photovoltaic effect.
The semiconducting properties of a semiconducting material may depend on the crystal structure of the material. Dopants, impurities and other defects may affect the resulting properties. Trace amounts of transition metals, oxygen, or carbon, for example, may affect conductivity or carrier lifetime.
The grain size and shape distribution often play an important part in the performance of semiconducting devices. Generally, a larger and more uniform grain size is desirable for semiconducting devices. For example, the electrical conductivity and efficiency of photovoltaic cells may be improved by increasing both the grain size and the uniformity of the grains.
For silicon-based solar photovoltaic cells, the silicon can be formed, for example, as an unsupported ingot, sheet or ribbon, or supported by forming the silicon on a substrate. Conventional methods for making unsupported and supported articles of semiconducting materials, such as silicon sheets, have several shortcomings.
Methods of making unsupported, i.e., without an integral substrate, thin semiconducting material sheets may be slow or wasteful of the semiconducting material feedstock. Methods by which unsupported single crystalline semiconducting materials are made include, for example, the Czochralski process, which may lead to significant kerf loss when the material is cut into thin sheets or wafers. Additional methods by which unsupported multicrystalline semiconducting materials are made include, for example, electromagnetic casting and ribbon growth techniques, which may be slow, producing about 1-2 cm/min for polycrystalline silicon ribbon growth technologies.
Supported semiconducting material sheets may be made less expensively, but the thin semiconducting material sheet may be limited by the substrate on which it is made, and the substrate may have to meet various process and application requirements, which may be conflicting.
Thus, there is a long-felt need in the industry for a method to make articles of semiconducting materials, which method would decrease the amount of impurities, decrease the amount of defects, improve crystal grain structure of the article of semiconducting material, reduce material waste, and/or increase the rate of production.
Useful methods for producing unsupported polycrystalline semiconducting materials are disclosed in commonly-owned U.S. Provisional Patent Application No. 61/067,679, filed Feb. 29, 2008, titled “METHOD OF MAKING AN UNSUPPORTED ARTICLE OF A PURE OR DOPED SEMICONDUCTING ELEMENT OR ALLOY,” and International Patent Application No. PCT/US09/01268, filed Feb. 27, 2009, titled “METHOD OF MAKING AN UNSUPPORTED ARTICLE OF A PURE OR DOPED SEMICONDUCTING ELEMENT OR ALLOY,” the disclosures of which are hereby incorporated by reference.
As described herein, the inventors have now discovered additional methods by which supported and unsupported articles of semiconducting materials may be made.